The present invention concerns an apparatus for making a sheet-formed fibrous product. The apparatus is intended for use in the so-called dry-forming method, a method by means of which air is used as carrier medium for the fibers for forming the desired sheet-formed fibrous product instead of water, which has been used as carrier medium by the wet method known for centuries.
By such dry-forming method an air stream containing a suspended defibrated cellulosic material is sucked up against an air pervious forming surface such as a wire net by means of a suction box placed onto the opposite side of the net and arranged for sucking air. Hereby a retention of the fibers occurs and thereby a forming of a coherent fiber layer on the forming surface and subsequently the fiber layer may be removed herefrom and be put through different after treatments for obtaining final products with desired physical and chemical qualities.
At an early stage of development in the dry-forming method a defibrator was used, such as a hammer mill, placed in such a way in relation to the forming surface that the defibrated material passed directly from the defibrator onto the forming surface. At a later stage a fiber distributor was placed between the defibrator and the forming surface, the distributor having a bottom in the form of a net having mesh openings which allow passage of single fibers and which restrain fiber agglomerates. The purpose of the fiber distributor is on the one hand to distribute the fiber material uniformly over the forming surface, such as an endless forming wire, and on the other hand to restrain non-defibrated fiber material or fiber agglomerates so that they are not carried onto the forming wire and result in an uneven fiber distribution in the fiber layer formed.
The known apparatuses of the above-mentioned type present the drawback that their velocity of production is substantially less than the velocity of production for the known apparatuses based on the wet method.
It has been tried to eliminate this drawback by increasing the velocity by which the forming wire is forwarded below the bottom of the fiber distributor and by increasing the fiber discharge from the fiber distributor. The aim hereby is to quickly deposit so many fibers onto the forming wire that they form a fiber layer in which the fibers touch each other and form a coherent sheet which may endure to be forwarded with high velocity without being damaged and which is uniform both in the machine direction and in the cross direction, i.e., without the so-called beach effect.
When manufacturing fiber products which substantially consist of cellulosic fibers, fiber distributors having a bottom consisting of a screen in the form of a wire net have been used, where the wires cross each other at acute angles, i.e., having quadratic or rectangular mesh openings. The meshes in the known fiber nets typically have the dimensions 2.5.times.2.5 mm.
In many situations it has been desirable to manufacture fiber products consisting of a mixture of cellulosic fibers and synthetic organic fibers, such a thermoplastic fibers, fibers having a thermoplastic surface, superabsorbent powder or granulate. By mixing such synthetic organic fibers and/or powder or granulate, fiber products having improved physical properties are obtained, e.g. having increased strength and/or larger ability of absorption.
Synthetic organic fibers, i.e. plastic fibers, necessarily have a greater length than cellulosic fibers and by using mixings of cellulosic fibers and organic synthetic fibers a tendency occurs that the synthetic organic fibers are retained at the bottom of the fiber distributor, whereas the cellulosic fibers pass through the distributor.
If this drawback is attempted to be overcome by increasing the largest dimension of the meshes so much that the synthetic organic fibers are able to pass through the bottom of the fiber distributor, the possibility, however, occurs that the non-defibrated lumps or agglomerates of cellulosic fibers pass through the bottom of the fiber distributor. This causes the fiber layer formed to have a great variation in fiber thickness, i.e., areas of close tangled fibers are formed, so-called fish eyes.
More specifically the present invention relates to a screen formed as a net-formed substantially plane bottom for a fiber distributor of an apparatus for manufacturing a sheet-formed fibrous product which apparatus further comprises a number of stirring wings which may be made to rotate around axes which are substantially perpendicular to the plane bottom, an endless forming wire having an upper and a lower run, the upper run being situated in a short distance from the under side of the bottom, and at least one suction box situation in such a way in relation to the upper run of the forming wire that fibers passing through the bottom are deposited on the upper run of the forming wire as a uniform coherent fiber layer, and means for removing the fiber layer from the forming wire in form of a sheet-formed fibrous product.
Apparatuses of the above-mentioned type are e.g., known from GB patent No. 2,008,638 and U.S. Pat. No. 4,494,278, said patents are hereby incorporated by reference. In these patents apparatuses are described comprising two or more built together fiber distributors.
Such apparatuses have widely been used for dry-forming of sheet-formed cellulosic fibrous products. These cellulosic fibrous products have a great ability to absorb water and are used widely for manufacturing tissues, diapers, incontinence products, sanitary towels and the like.
The object of the invention is to provide an apparatus of the above-mentioned type which makes it possible to manufacture uniform sheet-formed fiber products from mixtures of cellulosic fibers and organic synthetic fibers and with high velocity.